Ok, so it turns out that the latest develop branch on github doesn't seem to have this issue for the Content block. It does, however, exist for the Feature block. In fact, if you insert a page link into a Feature block, the full (absolute) URL is saved to the database in the form:

i'm also confused about this.
after completing a site i moved the dev-site to the clients server and the links created in content blocks point still to my dev-server. this is annoying. i had to relink everything created in content blocks. first i thought it was a mistake done by the editor at my clients firm. but it turns out the editor is not the culprit.

I asked because I'm pretty sure the issue cropped up for me in a pre-5.7.5.2 release. It seems to be resolved for Content blocks in C5.7.5.2+ (although there's an issue with Feature blocks storing the full URL in the DB).